Bushing for crawler belt and method of manufacture

ABSTRACT

Crawler belt bushings and their producing methods, with which productivity and cost performance can be improved over the carburization treatment and the induction hardening treatment. After a workpiece made from steel is heated from its outer circumferential surface such that at least the surface temperature of the inner circumferential surface of the workpiece is raised to a quenching temperature, a series of quenching operation is performed to form quench hardened layers which extend toward the core from the inner and outer circumferential surfaces respectively and a soft, imperfectly hardened layer between these quench hardened layers. The quenching operation comprises: cooling the workpiece from the inner circumferential surface; cooling the workpiece from the inner circumferential surface while heating from the outer circumferential surface; and cooling the workpiece from the outer circumferential surface.

TECHNICAL FIELD

[0001] The present invention relates to bushings for use in the crawlerbelts of construction vehicles such as bulldozers and their producingmethods. The invention more particularly relates to bushings for crawlerbelts excellent in wear resistance and impact fatigue resistance andsimple methods for producing them at low cost.

BACKGROUND ART

[0002] As shown in FIG. 36, a crawler belt 51 for construction vehiclesetc. is composed of various parts. A crawler belt bushing 52 meshes withsprocket teeth for transmitting rotating movement from final reductiongears and functions to rotate the crawler belt 51. As constructionvehicles are operated in soil and rock, crawler belt bushings need wearresistance at the inner and outer circumferential surfaces. Vehiclesalso travel, running over and colliding against soil and rock, crawlerbelt bushings need tremendously high strength and toughness. To meetthese requirements, there have been proposed the following producingmethods for crawler belt bushings.

[0003] (1) A case hardening steel is carburized to form very hardmartensite on its inner and outer circumferential surface layers,thereby ensuring wear resistance and strength (e.g. Japanese PatentPublication (KOKOKU) No. 52-34806 (1977)).

[0004] (2) A medium carbon steel is used as a bushing material. Thebushing material is thermally refined and its inner and outercircumferential surfaces are respectively induction hardened to formvery hard martensite thereon. After hardened by induction hardening fromthe outer circumferential surface, the bushing material is inductionhardened from the inner circumferential surface, so that a V-shapedhardened layer comprising tempered martensite is formed between theinner and outer hardened layers, thereby ensuring wear resistance andstrength (Japanese Patent Publication (KOKOKU) No. 63-16314).

[0005] (3) A medium carbon steel, whose hardenability is carefullycontrolled by precise adjustment on its chemical composition, is usedfor the bushing. Such a steel is heated in a furnace at a temperature of800° C. or more and then rapidly cooled thereby controlling the hardeneddepths of the inner and outer circumferential surfaces to ensure wearresistance and strength.

[0006] FIGS. 37(a), (b) and (c) schematically show typical hardeningpatterns for the bushings manufactured by the above conventionalmethods, and FIG. 37(d) shows the distributions of hardness in the crosssections of these bushings. All of the distributions indicate that thereis formed a soft layer at the wall core of the bushing.

[0007] These methods, however, reveal their own drawbacks. Thecarburization method (1) consumes considerable carburization time andhas the economical problem of using large amounts of carburizing gas.When producing large-sized crawler belt bushings having great thicknessfor example, a great hardened depth is required in order to ensurestrength and wear resistance, which gives rise to a decrease inproductivity and to increased cost. Further, since it takes long time tocarburize and heat the inner and outer circumferential surfaces, therewill be formed a grain boundary oxidized layer and imperfectly hardenedlayer having a thickness of several tens of μm, which causes a decreasein fatigue strength and impairs impact resistance properties.

[0008] The induction hardening method (2) is improved over thecarburization method (1) in terms of cost, but involves thermal refiningtreatment in order to assure hardness prior to the induction hardeningand double quenching of the inner and outer circumferential surfaces, sothat this method has proved to remain a costly heating treatment.Further, when induction hardening the inner circumferential surface of asmall-diameter tubular part, an inner circumference heating coil isneeded and such a heating coil is difficult to manufacture. Therefore,in many cases, the inner circumferential surface of a tubular part ishardened by the above carburization treatment, resulting in high cost.

[0009] The outer circumferential surface of a bushing in use issubjected to severe wearing conditions due to soil and rock. To increasethe wear life of the bushing, the quench hardened layer at the outercircumferential surface is preferably more deepened. To this end, therehas been made an attempt in Japanese Patent Publication (KOKOKU) No.63-16314 (1988). According to this publication, after the outercircumferential surface of a steel is once hardened deeply by inductionhardening from the outer circumferential surface, the innercircumferential surface is shallowly hardened by induction hardeningfrom the inner circumferential surface, and a soft layer is formedbetween these hardened layers by high temperature tempering. In anycase, double induction hardening is involved, which is disadvantageousin terms of productivity and economy. In the method of this publication,it is necessary to control the induction hardening from the outercircumferential so as to restrict the hardness of the innercircumferential surface to H_(RC) 40, thereby preventing quenching crackduring the later induction hardening of the inner circumferentialsurface. For applying this method to a comparatively thin tubular part(e.g., crawler belt bushing), it is necessary to control the temperatureof the inner circumferential surface with high precision, during theinduction heating from the outer circumferential surface and/or tocontrol the hardenability (DI value) of the steel material to be used.As a result, the technical difficulty in deepening the outercircumferential surface hardened layer and increased material cost areinevitable.

[0010] Japanese Patent Publication (KOKOKU) No. 1-37453 discloses aquite economical heating process for producing through hardened crawlerbushings. This process uses a medium carbon steel for the bushing. Whilescan induction heating is carried out from the outer circumferentialsurface of the bushing, the bushing is cooled from the outercircumferential surface, so that the bushing is hardened across itsentire thickness. For through hardening the bushing across its entirethickness by cooling from the outer circumferential surface only, highhardenability is required when this process is applied to the productionof thick, large-sized crawler belt bushings, which inevitably entails anincrease in cost. In addition, in view of susceptibility to quenchingcrack during cooling, the steel material that can be used in thisprocess is limited to medium carbon low alloy steels having carboncontents of 0.5 wt % or less. As a result, it becomes difficult toimprove the wear resistance of the outer circumferential surface of acrawler belt bushing.

[0011] The hardening process (3) has overcome the cost problemspresented by the processes (1) and (2), but presents other problems.Specifically, it is necessary for this process to accurately andnarrowly control the hardenability (DI value) of the steel materialused, by correctly grasping the relationship between the thickness ofthe steel and the cooling rate. This process has a problem in theavailability of materials to be used. In addition, as the thickness ofthe busing becomes smaller, the bushing is more through hardened acrossits entire thickness, causing higher tensile residual stress at theinner and outer circumferential surfaces. This could be a cause forquenching crack and a considerable decrease in fatigue strength. Toavoid this problem, the DI value should be decreased and as a result,the steel materials which meet this requirement will not be commerciallyavailable. Further, if steels having small DI values are used, theresultant bushings will have variations in hardness.

[0012] The present invention has been directed to overcoming theabove-described problems and therefore the prime object of the inventionis to provide a crawler belt bushing and its producing method, improvedover the products and processes of the above conventional carburizationand induction hardening in terms of productivity and cost. This crawlerbelt busing is produced by heating a tubular bushing workpiece made ofsteel to a quenching temperature and then quenching the workpiece in aseries of quenching operation, without causing quenching crack, so thatthe workpiece is quench hardened across its entire thickness.

[0013] The invention also aims to provide a long wear life, toughcrawler belt bushing and its producing method, improved over theproducts and processes of the above conventional carburization andinduction hardening in terms of productivity and cost. This busing isproduced by heating a tubular bushing workpiece made of steel to aquenching temperature and then applying a series of quenching operationto this workpiece, the operation comprising: advance cooling of theworkpiece from the inner circumferential surface and cooling of theworkpiece from the outer circumferential surface after waiting a certaintime. With this process, quench hardened layers are formed at the innerand outer circumferential surfaces respectively, such that at least thedepth of the outer circumferential surface hardened layer is greaterthan that of the inner circumferential surface hardened layer.

[0014] The invention provides a crawler belt bushing producing methodapplicable to inexpensive steel materials having higher commercialavailability than the above-described through-hardening process from theouter circumferential surface only, by achieving through hardening bycooling from both inner and outer circumferential surfaces althoughthere is a difference between the starting time for the innercircumferential surface cooling and the starting time for the outercircumferential surface cooling.

[0015] The invention also aims to provide a crawler belt bushing and itsproducing method, improved over the products and processes of the aboveconventional carburization and induction hardening in terms ofproductivity and cost, this busing being produced by the followingprocess. A tubular bushing workpiece made of steel is heated by scaninduction heating from its outer circumferential surface such that atleast the temperature of the inner circumferential surface of theworkpiece is raised to a quenching temperature. Then, a series ofquenching operation is carried out as follows. While firstly startingcooling from the inner circumferential surface, the outercircumferential surface is heated by induction heating to restrict thecooling of the outer circumferential surface occurring from the innercircumferential surface. After waiting a certain time in order toprevent the core of the workpiece from being fully hardened, coolingfrom the outer circumferential surface is started. Through these steps,quench hardened layers are formed at the inner and outer circumferentialsurfaces respectively.

[0016] For ensuring formation of a quench hardened layer at the innercircumferential surface, the advance cooling from the innercircumferential surface, the induction heating from the outercircumferential surface and the later cooling from the outercircumferential surface are carried out as described earlier, therebyforming a soft layer within the core of the bushing at a cross-sectionalposition closer to the inner circumferential surface. With thisarrangement, a soft layer can be formed in the core, even when using, asa bushing material, a steel having such a great DI value that the steelis through hardened by cooling from the inner circumferential surfaceonly. In consequence, quenching crack can be prevented and the hardeneddepth of the outer circumferential surface can be greater than thehardened dept of the inner circumferential surface, so that a crawlerbelt bushing improved in wear resistance and fatigue strength and itsproducing method can be provided.

[0017] The above induction heating/hardening method of the invention isapplicable to tubular parts similar to crawler belt bushings. Since thismethod does not need to carry out heat hardening from the innercircumferential surface and therefore does not use an induction heatingcoil for an inner circumferential surface, small-diameter tubular parts(e.g., small-sized crawler belt bushings) and thin tubular parts (e.g.,crawler belt bushings) can be produced at low cost.

[0018] Further, since a soft layer can be formed within the wall of thecrawler belt bushing and quenching crack can be prevented even whenusing a steel having high hardenability as has been noted above, theinvention can provide a crawler belt bushing having high wear resistanceat its outer circumferential surface and its producing method, byhardening a steel material which has comparatively high hardenabilityand a carbon content of about 2.0 wt % and is composed of austenitecontaining cementite grains dispersed therein. It is preferable tosubstantially uniformly disperse cementite in the workpiece by thermalrefining or the like, prior to starting of the hardening operation.

DISCLOSURE OF THE INVENTION

[0019] According to a first aspect of the invention, there is provided acrawler belt bushing producing method, wherein a workpiece of a crawlerbelt bushing made of a medium or high carbon steel or a medium or highcarbon low alloy steel is heated to a quenching temperature or more; byuse of a hardening system which can independently start outercircumferential surface cooling and inner circumferential surfacecooling, the workpiece is first cooled from either one of its outercircumferential surface and inner circumferential surface and thencooling is carried out from the other circumferential surface so thatthe workpiece is hardened through its entire thickness; and then theworkpiece is tempered.

[0020] In the above method, after the bushing workpiece is heated to aquenching temperature, a series of quenching operation is carried outusing a hardening system which can independently start outercircumferential surface cooling and inner circumferential surfacecooling and using a cooling medium such as water, water-solublequenching liquid or oil. The quenching operation comprises the steps of(i) advance cooling from either one of the inner and outercircumferential surfaces to reduce heat capacity at the core of theworkpiece to provide a heat gradient and (ii) cooling from the othercircumferential surface which is started after waiting for a certaintime to reduce tensile stress due to possible heat and deformationstresses generated during quenching and therefore to reducesusceptibility to quenching crack caused by through hardening. With thismethod, possible quenching crack due to through hardening can beprevented even when using a steel which has a high carbon content andhigh susceptibility to quenching crack and is usually through hardenedby simultaneous cooling from the inner and outer circumferentialsurfaces. Accordingly, a bushing having improved wear life at the outercircumferential surface can be economically produced with this method.

[0021] By using a medium or high carbon steel having a carbon content of0.35 wt % to 1.5 wt % as a steel material for the bushing and increasingthe hardness of the quench hardened layer at the outer circumferentialsurface to be equal or more than those of carburization hardenedbushings, the crawler belt bushing improved in wear resistance and wearlife can be produced at low cost. The alloy composition of a steel is afactor that determines the hardenability of the steel. The alloycompositions of the steels used in the invention are determined by thelower limit of DI value with which the bushing is through hardened bysimultaneous cooling from the inner and outer circumferential surfaces.As has been noted above, the invention is basically directed to coolingfrom the inner and outer circumferential surfaces and therefore it canuse steel materials less expensive than medium carbon low alloy steelswhich can be through hardened by cooling from the outer circumferentialsurface only. This advantage leads to considerable cost reduction andmakes the invention applicable to the production of large-sized, thickcrawler belt bushings.

[0022] While the wear resistance of the outer circumferential surface ofthe bushing is ensured by use of medium and high carbon steels, theimpact resistance (toughness) of the bushing is obtained by allowingself-tempering of the inner circumference by finishing the cooling fromthe inner circumferential surface at an early time. The impactresistance of the bushing may be obtained by induction tempering theworkpiece from the inner circumferential surface subsequently to thequenching operation so that the hardened depth of the innercircumferential surface is adjusted to 450 to 600 Hv. Accordingly, thecrawler belt bushing having wear resistance and impact resistance ashigh as those of carburization hardened layers while keeping highhardness at the quench hardened layer of the outer circumferentialsurface can be produced at low cost by the invention.

[0023] The invention is based on the thermal operation in which aftersubstantially uniform, entire heating of the bushing, cooling fromeither one of the inner or outer circumferential surfaces is firststarted and then cooling from the other circumferential surface isstared, so that hardening can be completed within one cycle ofoperation. Unlike the conventional induction quenching, the inventiondoes not need to do adjustment twice, that is, hardened depth adjustmentfor the inner circumferential surface and for the outer circumferentialsurface, and does not need to heat/quench the workpiece from the innerand outer circumferential surfaces separately, so that the inventionprovides high productivity. The heating method is not limited toinduction heating and furnace heating, but induction heating ispreferred when taking into account productivity, system cost and energyefficiency.

[0024] Further, the hardening process of the invention employs aquenching system in which starting time for inner circumferentialsurface cooling and starting time for outer circumferential surfacecooling can be independently determined. Uneven cooling is likely tooccur when cooling the inner circumferential surface of a tubular body,and therefore the invention preferably employs jet cooling such as waterspraying or oil spraying. In order to prevent the cooling medium for theinner circumferential surface from touching the outer circumferenceduring the advance cooling from the inner circumferential surface, it ispreferable to set the spray at an appropriate angle in consideration ofthe flows of the cooling media as shown in FIG. 1 or to provide apartition (shielding plate) as indicated by A in FIG. 1.

[0025] In the case incorporating furnace heating, when a multiplicity ofbushings is cooled by employing the above-described advance cooling fromthe inner or outer circumferential surface, it is preferable that thebushings be aligned with their adjacent end faces in contact with eachother, like one steel pipe as shown in FIGS. 2(a) and 2(b), and thentheir inner circumferential surfaces and outer circumferential surfacesbe respectively cooled by independently controlling innercircumferential surface cooling water 2 and outer circumferentialsurface cooling water 3. It should be noted that the cooling water 2 andthe cooling water 3 are shielded from each other with a shielding plate4. In the case shown in FIGS. 2(b) and 2(c), there is disposed an innercircumferential surface cooling nozzle 5.

[0026] The time difference hardening method, in which while heating apart of the bushing by scan heating with an induction coil, coolingfrom, for instance, the inner circumferential surface starts in advanceof cooling from the outer circumferential surface, does not involve alarge hardening system and has a high degree of freedom in production. Apreferred arrangement to effect this method is shown in FIG. 3 in whichshielding plates 4, 4′ are positioned at the upper and lower end facesof the bushing 1 respectively and the inner circumference surfacecooling nozzle 5 and the outer circumferential surface cooling nozzle 6are designed such that the nozzle 5 firstly cools an induction heatedzone and a specified timer later, the nozzle 6 starts cooling.Preferably, the scan hardening is carried out by the relative movementof an induction heating coil 7, the nozzle 5 and the nozzle 6 along theaxis of the bushing 1 and such a relative movement is preferably carriedout while the bushing 1 being rotated. It is a matter of course thatwhen cooling the outer circumferential surface first, the coolingnozzles should be arranged oppositely to the above arrangement.

[0027] As has been described above, the invention is designed such that(1) a bushing is substantially uniformly heated by induction heating orfurnace heating and (2) cooling from the inner or outer circumferentialsurface of the bushing is started in advance of (3) cooling from theouter or inner circumferential surface to eliminate susceptibility toquenching crack, by use of a cooling medium such as oil or water. Withthis arrangement, the crawler belt bushing made of inexpensive, lowhardenability, medium or high carbon steel can be quench-hardenedthroughout its entire thickness within one cycle of operation and thebushing thus hardened is improved in the wear life of the outercircumferential surface and production cost.

[0028] The formation of the deeply hardened layer having high hardness,high wear resistance and carbon content equal to or higher than those ofcarburized bushings leads to considerable improvements in the wearresistance and wear life of the resultant bushing. Further, theformation of the soft layer in the core ensures toughness as high asthat of the conventional bushings, whereas tempering of the innercircumferential surface at higher temperatures increases the toughnessof the inner circumferential surface layer. These all lead toimprovements in the impact strength and functions of the bushing.

[0029] According to a second aspect of the invention, there is provideda crawler belt bushing producing method wherein, after a workpiece of acrawler belt bushing made of steel is heated to a quenching temperature,

[0030] (a) the cooling rate of the outer circumferential surface of theworkpiece is increased by first cooling of the workpiece from its innercircumferential surface in order to reduce heat capacity at the core ofthe workpiece and by second cooling of the workpiece from its outercircumferential surface which is started a certain time after the firstcooling and/or

[0031] (b) the cooling rate of the outer circumferential surface of theworkpiece is increased by first cooling of the workpiece from its innercircumferential surface in order to partially make the core of theworkpiece unhardenable by utilizing the mass effect of the wall of theworkpiece and by second cooling of the workpiece from its outercircumferential surface which is started a certain time after the firstcooling,

[0032] whereby a soft layer is formed within the core of the workpieceat a cross-sectional position closer to the inner circumferentialsurface and the hardened depth of the outer circumferential surface ismade to be greater than the hardened depth of the inner circumferentialsurface,

[0033] these processes (a) and (b) being carried out within one cycle ofquenching operation, using a hardening system capable of performinginner circumferential surface cooling and outer circumferential surfacecooling.

[0034] In the invention having the above feature, a soft layer is formedwithin the wall core at a cross-sectional position closer to the innercircumferential surface, so that the bushing has a U-shaped hardnessdistribution. With this arrangement, when using, as the material of thebushing, a steel which is usually through hardened by simultaneouscooling from the inner and outer circumferential surfaces, the bushingcan be prevented from quenching crack. Further, the hardened depth ofthe outer circumferential surface is made to be greater than thehardened depth of the inner circumferential surface, which entails animprovement in the wear life of the outer circumferential surface of thebushing and enables economical manufacture.

[0035] The hardenabilities of the steels to which the invention isapplicable are dependent on their alloy compositions. Steels having awider variety of alloy compositions can be used by employing a widerrange of DI values with which the hardened depth achieved by coolingfrom the inner circumferential surface only is about one half thethickness of the bushing or less, even though the bushing is throughhardened by simultaneous cooling from the inner and outercircumferential surfaces. With this arrangement, commercially available,inexpensive steel materials can be used in the invention and a hardeneddepth one-half the thickness of the bushing or more can be easilyensured for the outer circumferential surface thereby highly improvingthe wear life of the outer circumference of the bushing.

[0036] The wear resistance of the outer circumferential surface of thebushing is improved by the arrangement in which after heating theworkpiece to a quenching temperature, quenching is carried out byadvance cooling from the inner circumferential surface and later coolingfrom the outer circumferential surface and in which induction temperingis carried out from the inner circumferential surface while the hardeneddepth of the outer circumferential surface being kept high, therebyincreasing particularly the toughness of the inner circumference surfacehardened layer. This enables the economical manufacture of the bushinghaving wear resistance and impact resistance equal to or higher thanthose of carburization hardened layers.

[0037] According to the invention, in one cycle of operation, (1) aworkpiece is substantially uniformly heated by induction heating orfurnace heating and then, (2) cooling from the inner circumferentialsurface of the workpiece is started in advance of (3) cooling from theouter circumferential surface, by use of a cooling medium such as oil orwater, whereby the hardened depth of the inner circumferential surfaceis made to be smaller than that of the outer circumferential surface andwhereby the cooling of the outer circumferential surface can beexpedited by the advance cooling from the inner circumferential surfaceto further deepen the hardened layer. With this arrangement, a softlayer can be formed at the core thereby preventing possible quenchingcrack, even when using a steel that is usually through hardened bysimultaneous cooling from the inner and outer circumferential surfaces.Further, the outer circumferential surface hardened layer can be made tobe deeper than the inner circumferential surface hardened layer, toimprove the wear life of the bushing. This, in consequence, brings aboutlots of benefits in economy.

[0038] In addition, the formation of the deeply hardened layer on theouter circumferential surface entails improvements in the wearresistance and wear life of the bushing, the hardened layer having highwear resistance, high hardness and carbon content substantially equal tothose of carburized bushings. Further, the formation of the soft layerin the core assures toughness equal to that of the conventionalbushings, and the high temperature tempering of the innercircumferential surface toughens the inner circumferential surfacelayer. These all contribute to improvements in the impact strength andfunctions of the bushing.

[0039] According to a third aspect of the invention, there is provided acrawler belt bushing producing method wherein, after a workpiece of acrawler belt bushing made of steel is induction heated from its outercircumferential surface such that at least the temperature of the innercircumferential surface of the workpiece is raised to a quenchingtemperature, a series of quenching operation comprising:

[0040] (a) firstly cooling the workpiece from the inner circumferentialsurface;

[0041] (b) cooing the workpiece from the inner circumferential surfacewhile heating the workpiece from the outer circumferential surface; and

[0042] (c) cooling the workpiece from the outer circumferential surface,

[0043] is performed so as to form quench hardened layers which extendtoward the wall core of the workpiece from the outer circumferentialsurface and from the inner circumferential surface respectively and forma soft, imperfectly hardened layer between these quench hardened layers.

[0044] According to a forth aspect of the invention, there is provided acrawler belt bushing producing method wherein, while a workpiece of acrawler belt bushing made of steel being heated from its outercircumferential surface by scan induction heating, using at least twovertically aligned, induction coils,

[0045] (a) the temperature of the inner circumferential surface of theworkpiece is raised to a quenching temperature equal to thetransformation temperature of A1, A3 or Acm or more;

[0046] (b) the workpiece is partially heated from the outercircumferential surface by the induction coils while carrying out firstcooling from the inner circumferential surface; and

[0047] (c) the workpiece is then cooled from the outer circumferentialsurface;

[0048] whereby the inner and outer circumferential surfaces are quenchhardened so as to be substantially martensitic, these processes (a), (b)and (c) being carried out at a certain position of the workpiece.

[0049] In the invention having the above feature, after the workpiece isheated from the outer circumferential surface by induction heating sothat the temperature of the inner circumferential surface is raised to aquenching temperature, a series of quenching operation is carried out inthe following way, using a cooling medium such as water, water-solublequenching liquid or quenching oil. In the quenching operation, coolingfrom the inner circumferential surface is carried out while partiallyheating the workpiece from the outer circumferential surface byinduction heating, thereby restricting the cooling of the outercircumference from the inside. Then, cooling from the outercircumferential surface is carried out after waiting a certain timewhich is long enough to disallow the inside of the wall to be fullyquenched by the cooling from the outer circumferential surface. Withthis arrangement, a soft, imperfectly hardened structure can be formedwithin the core and a satisfactorily hardened depth can be obtained atthe outer circumferential surface by the later cooling from the outercircumferential surface that starts after waiting a certain time. This,in consequence, enables the economical producing method for the crawlerbelt bushing having quench hardened layers at the inner and outercircumferential surfaces.

[0050] When using a steel which is usually through hardened by thesimultaneous cooling from the inner and outer circumferential surfacesor by cooling from the inner circumferential surface only, the inventioncan form a soft layer within the core at a cross-sectional positioncloser to the inner circumferential surface, because of the inductionheating from the outer circumference that is carried out during theadvance cooling from the inner circumferential surface. Therefore, inmany cases, there is substantially no need to control the hardenabilityof the steel to be used in the invention, which enables use ofinexpensive, commercially available steel materials. This leads to costreduction.

[0051] It should be noted that the advance cooling from the innercircumferential surface in the invention is intended for reducing heatcapacity in the core of the bushing. The reduction in heat capacity, inturn, expedites the later cooling from the outer circumferential surfaceso that the outer circumferential surface hardened layer can be moredeepened than the inner circumferential surface hardened layer. Theproducing method incorporating the above principle is particularlysuited for the production of crawler belt bushings having excellent wearlife at their outer circumferential surfaces.

[0052] For further deepening the quench hardened layer of the outercircumferential surface, a steel, which has at least such hardenability(DI value) that the hardened depth obtained by hardening from the outercircumferential surface only is one half the thickness of the bushing,is used for the bushing; the bushing is hardened to a depth equal to thewear critical point (about one half the thickness of the bushing); andthe above-described soft hardened layer is formed in the wall core at aposition closer to the inner circumferential surface. The resultantbushing is superior in strength, toughness, and wear life.

[0053] As the method of induction heating the bushing from the outercircumferential surface, an entire heating method or a scan heatingmethod may be employed. As shown in FIG. 4, in the entire heatingmethod, when heating the outer circumferential surface with highfrequency coil 2 during cooling from the inner circumferential surfacewith an inner circumferential surface cooling nozzle 4, the hardeneddepths of the inner and outer circumferential surfaces can be controlledas required by controlling the electric power of the high frequency coil2. The cooling from the outer circumferential surface may be carriedout, for example, by moving outer circumferential surface cooling nozzle5 from underneath after moving the high frequency coil 2 upward. Anotherarrangement is such that the bushing is cooled by a coolant jettedthrough the clearances between the inductors of the high frequency coil2.

[0054] As shown in FIG. 5, in the scan heating method, a wide inductioncoil or, more preferably, two or more vertically aligned induction coils(high frequency coils) 8, 9 are used. These induction heating coils arearranged to perform induction heating so as to prevent the outercircumferential surface from being cooled by the cooling from the innercircumferential surface. In this way, the core of the wall will not befully hardened.

[0055] The hardened depths of the inner and outer circumferentialsurfaces can be easily controlled in the following way. Taking intoaccount the relative moving speed of the induction coils and thebushing, the induction heating of the outer circumferential surfaceduring the cooling from the inner circumferential surface is mainlyperformed by the second high frequency coil 9 and the distance betweenthe cooling position of the outer circumference of the bushing cooled bythe outer circumferential surface cooling nozzle 11 and the position ofthe second high frequency coil 9 is adjusted to control the time to betaken for induction heating, that is, the time after the cooling fromthe inner circumferential surface is started until the cooling from theouter circumferential surface is started.

[0056] These induction heating/hardening methods do not need to heat andquench the bushing from the inner circumferential surface and thereforeenables the economical production of small-diameter tubular parts (e.g.,small-sized crawler belt bushings) and extremely thin tubular parts(crawler belt bushings), for which induction heating coils for an innercircumferential surface are difficult to produce.

[0057] In view of wear resistance and strength, the carbon contents ofthe steels used in this example are preferably 0.35 to 2.0 wt %, so thatthe hardened bushing has a hardness of H_(RC) 50 or more and is improvedin the hardness of the outer circumferential surface hardened layer.

[0058] It is generally known that increasing of the carbon content of asteel to be used is effective in economically manufacturing crawler beltbushings having superior wear resistance and wear life. The conventionalinduction hardening methods are not applied to steels having carboncontents of 0.55 wt % or more because there is a high risk of quenchingcrack. Thanks to the foregoing heating/cooling principle, the inventioncan prevent quenching crack and, therefore, can economically produce acrawler belt bushing having superior wear resistance at the outercircumferential surface by hardening the bushing in an austenite statein which cementite grains are dispersed, even when steel materialshaving comparatively good hardenability and carbon contents as high as2.0 wt % are used. Note that it is preferable to treat the bushing, forexample, by thermal refining, prior to the hardening operation, in orderthat cementite grains are substantially evenly dispersed in thestructure of the bushing.

[0059] For particularly improving the wear resistance of the outercircumferential surface, after the bushing workpiece is heated to aquenching temperature, quenching is carried out by the above methodwherein the inner circumferential surface is first cooled, and while thehardened depth of the outer circumferential surface being kept high, theinner circumferential surface is induction tempered, therebyparticularly increasing the toughness of the inner circumferentialsurface hardened layer. This makes it possible to economically produce acrawler belt bushing having wear resistance and impact resistance equalto or higher than those of carburization hardened layers.

[0060] One of the features of the invention resides in the thermaloperation in which while the scan induction heating being carried out,cooling from the inner circumferential surface is first started andthen, the outer circumferential surface is cooled, so thatheating/quenching is completed within one cycle of operation. Therefore,there is no need to separately perform hardened depth adjustment andseparately perform heating/quenching for the inner and outercircumferential surfaces, which results in high productivity, savings inequipment investment cost, and improved energy efficiency.

[0061] In the above hardening method, the hardened depth of the innercircumferential surface can be made to be greater than the hardeneddepth of the outer circumferential surface, by controlling the output ofthe outer circumferential surface heating nozzle during the advancecooling from the inner circumferential surface and then carrying out theouter circumferential surface cooling. Therefore, this method issuitably applied to the production of high strength steel pipes used fordelivering slurry etc., which require high wear resistance at theirinner circumferences.

[0062] In view of possible uneven cooling, the suitable cooling methodfor the inner circumferential surface is jet cooling such as waterspraying or oil spraying. It is preferable to set the spray at an anglein consideration of the flows of the cooling media as shown in FIGS. 4and 5 or to provide a partition such as the shielding plate 1 shown inFIG. 4, in order that the cooling medium for cooling the innercircumferential surface does not interfere with the outer circumferenceduring the advance cooling from the inner circumferential surface.

[0063] A multiplicity of bushings can be treated by applying the abovehardening method to the bushings aligned with their adjacent end facesin contact with each other as described earlier.

[0064] The time difference hardening method, in which the bushing ispartially heated by scan heating with the induction coil; the innercircumferential surface is first cooled while heating is carried outwith the high frequency coils so as to prevent the outer circumferentialsurface from being cooled by the cooling from the inner circumferentialsurface; and then, cooling from the outer circumferential surface isstarted, does not involve a large hardening system and has a high degreeof freedom in the production. In this case, there may be provided theshielding plate 1 and shielding cap 6 near the lower and upper end facesof the bushing respectively, as shown in FIG. 4. Preferably, the innercircumferential surface cooling nozzle 4 first heats the inductionheating zone and a certain time later, cooling from the outercircumferential surface is started, and while the bushing 3 beingrotated, the induction heating coil 2, the inner and outercircumferential surface cooling nozzles 4, 5 are relatively moved alongthe axis of the bushing, thereby performing the scan hardening.

[0065] According to the invention, the scan induction heating, coolingfrom the inner circumferential surface and cooling from the outercircumferential surface are subsequently started with time differences,whereby the bushing having quenched hardened layers at the inner andouter circumferential surfaces and the soft layer at the core, or thebushing having the outer circumferential surface hardened layer that isdeeper than the inner circumferential surface hardened layer can bemanufactured by one cycle of operation. With this arrangement, quenchingcrack can be prevented even when using a steel which is usually throughhardened by the simultaneous cooling from the inner and outercircumferential surfaces. Further, the carbon contents of the steels tobe used in the invention can be increased. This leads to improvement inwear life and enables an economical production method for long-lifebushings.

[0066] In the invention, it is preferable to simultaneously performcooling from the inner circumferential surface and from the outercircumferential surface at least within a specified zone close to theupper end face and the lower end face, respectively, of the bushing. Bycontrolling the striking position of the cooling medium for the innercircumferential surface and the striking position of the cooling mediumfor the outer circumferential surface to be equal to each other at leastwithin a specified zone of the bushing during the advance cooling fromthe inner circumferential surface, a hardened layer can be easily formedon the end face, so that the resulting bushing has an ideal profile inwhich the imperfectly hardened layer is enclosed by the hardened layersof the inner and outer circumferential surfaces and the hardened layerof the end face.

BRIEF EXPLANATION OF THE DRAWINGS

[0067]FIG. 1 is a sectional view of a quenching system.

[0068] FIGS. 2(a) and 2(b) are sectional views each showing a quenchingsystem for treating a multiplicity of bushings and

[0069]FIG. 2(c) is a longitudinal section of the system shown in FIG.2(b).

[0070]FIG. 3 is a sectional view of a hardening system employinginduction heating coils.

[0071]FIG. 4 is a schematic general view of an entire heating inductionhardening system.

[0072]FIG. 5 is a schematic view of a scan induction hardening system.

[0073]FIG. 6 is a sectional view showing the configuration of a bushingsample.

[0074]FIG. 7 is a graph showing the relationship between the degree ofthrough hardening of bushings having the dimension D and the frequencyof quenching crack in bushings.

[0075]FIG. 8 is a graph showing the distributions of hardness inbushings having the dimension D and the composition No. 1 and treated bythe time difference hardening.

[0076]FIG. 9 is a graph showing the distributions of hardness inbushings having the dimension D and the composition No. 2 and treated bythe time difference hardening.

[0077]FIG. 10 is a graph showing the distributions of hardness inbushings having the dimension D and the composition No. 4 and treated bythe time difference hardening.

[0078]FIG. 11 is a graph showing the distributions of hardness inbushings having the dimension D and the composition No. 5 and treated bythe time difference hardening.

[0079]FIG. 12 is a graph showing the distributions of hardness inbushings having the dimension D and the composition No. 6 and treated bythe time difference hardening.

[0080]FIG. 13 is a graph showing the distributions of hardness inbushings having the dimension A and the composition No. 7 and treated bythe time difference hardening.

[0081]FIG. 14 is a graph showing the distributions of hardness inbushings having the dimension B and the composition No. 8 and treated bythe time difference hardening.

[0082]FIG. 15 is a graph showing the distributions of hardness inbushings having the dimension C and the composition Nos. 7, 8 andtreated by the time difference hardening.

[0083]FIG. 16 is a graph showing the distributions of hardness inbushings having the dimension D and the composition No. 2 and treated bythe time difference hardening.

[0084]FIG. 17 is a graph showing the distributions of hardness inbushings having the dimension D and the composition No. 3 and treated bythe time difference hardening.

[0085]FIG. 18 is a graph showing the distributions of hardness inbushings having the dimension D and the composition No. 4 and treated bythe time difference hardening.

[0086]FIG. 19 is a graph showing the relationship between the hardeneddepths of the inner and outer circumferential surfaces of a bushinghaving the dimension C and the lead time of advance cooling from theinner circumferential surface.

[0087]FIG. 20 is a graph showing the relationship between the hardeneddepths of the inner and outer circumferential surfaces of a bushinghaving the dimension C and the lead time of advance cooling from theouter circumferential surface.

[0088]FIG. 21 is a graph showing test results relating to the wear ofthe outer circumferences of bushings in service.

[0089]FIG. 22 shows a method for testing collapse fatigue.

[0090]FIG. 23 is a graph showing the results of collapse fatigue tests.

[0091]FIG. 24 shows a method for testing impact fatigue.

[0092]FIG. 25 is a graph (1) showing the results of impact fatiguetests.

[0093]FIG. 26 is a graph (2) showing the results of impact fatiguetests.

[0094]FIG. 27 is a graph showing the distribution of hardness in thewall cross section of a bushing tempered at 140° C. for one hour afterthe scan induction heating/time difference quenching.

[0095]FIG. 28 is a graph showing the results of the scan inductionhardening of bushings having the composition No. 4.

[0096]FIG. 29 is a graph showing the results of the scan inductionhardening of bushings having the composition No. 11.

[0097]FIG. 30 is a graph showing the results of the scan inductionhardening of bushings having the composition No. 12.

[0098]FIG. 31 is a graph (1) showing the result of the entire inductionhardening of a bushing having the composition No. 11.

[0099]FIG. 32 is a graph (2) showing the result of the entire inductionhardening of a bushing having the composition No. 11.

[0100]FIG. 33 is a graph showing the results of impact fatigue tests.

[0101] FIGS. 34(a) to 34(f) show the steps of the scan inductionhardening in Example 4.

[0102] FIGS. 35(a) to 35(c) show the effects of Example 4.

[0103]FIG. 36 is an exploded perspective view of a crawler belt bushing.

[0104] FIGS. 37(a), 37(b) and 37(c) are diagrams showing typicalhardening patterns for bushings produced by the conventional methods and

[0105]FIG. 37(d) is a graph showing the distributions of hardness in thecross sections of these bushings.

BEST MODE FOR CARRYING OUT THE INVENTION

[0106] Referring now to the drawings, crawler belt bushings and theirproducing methods will be explained according to preferred embodimentsof the invention.

EXAMPLE 1

[0107] TABLE 1 shows the steel compositions of the bushings used inExample 1. FIG. 6 shows the configuration of the bushings used in thisexample and TABLE 2 shows the dimension of each bushing. Heating forhardening was carried out in a furnace in a neutral atmosphere and thespray quenching system shown in FIG. 1 was used. The spray quenchingsystem comprises a spray for cooling the inner circumferential surfaceof a bushing and another spray for cooing the outer circumferentialsurface. These sprays are independently controlled so as to startcooling operation at different times. The spray for the innercircumferential surface is designed to have an adequate jetting anglewith respect to a normal line of the inner circumferential surface, soas to allow water present in the inner circumference to flow toward thelower part of the bushing without being trapped. Disposed near the lowerend of the bushing is a shield plate for dividing a flow of coolingwater used for inner circumferential surface cooling from a flow ofcooling used for outer circumferential surface cooling. Disposed nearthe upper end is a cap for dividing a flow of cooling water used forinner circumferential surface cooling from a flow of cooling water usedfor outer circumferential surface cooling. TABLE 1 STEEL COMPOSITION (WT%) C Si Mn Cr P S Al Dl No 1 0.58 0.18 0.72 — 0.015 0.015 0.03 0.91 No 20.54 0.23 0.81 — 0.016 0.016 0.042 0.98 No 3 0.62 0.28 0.98 — 0.0170.018 0.041 1.25 No 4 0.61 0.19 1.17 — 0.018 0.018 0.037 1.35 No 5 0.640.24 1.32 — 0.019 0.018 0.037 1.63 No 6 1.34 0.18 0.68 0.21 0.017 0.0150.032 No 7 0.47 0.09 0.34 — — — — 0.49 No 8 0.53 0.23 0.48 — 0.008 0.008— 0.69 No 9 0.57 0.23 0.81 — 0.021 0.017 0.038 1.01 No 10 0.72 0.21 0.47— 0.025 0.02 0.014 0.81

[0108] TABLE 2 D1 D2 t L A 41 24.4 8.3 81 B 47 28.2 9.4 94 C 59 38 10.5138 D 79 50 14.5 202

[0109] Hardening operation was basically carried out under the aboveconditions. Specifically, after intensely heating at 850° C. for 30minutes in a furnace, each crawler belt bushing was quickly loaded inthe quenching system shown in FIG. 1. Quenching was carried out by innercircumferential surface cooling and outer circumferential surfacecooling under their respective specified conditions and then, followedby low temperature tempering at 140° C. for 3 hours. It should be notedthat “entire induction heating” from the outer circumferential surfaceof the bushing was incorporated as a part of the heating process.

[0110]FIG. 7 shows the relationship between the degree of throughhardening and the frequency of quenching crack when the crawler beltbushings (Dimension D) manufactured from the steel materials Nos. 1 to 4were quenched by simultaneous cooling from the inner and outercircumferential surfaces. Surface residual stress is plotted as theordinate while the gradient of hardness in the outer circumferentialsurface as the abscissa. FIG. 7 also indicates the number of bushingsout of 10 bushings, in which quenching crack occurred. As seen from thisfigure, no quenching crack occurred in the bushing made of the steel No.1 and having a soft layer at its core whereas quenching crack occurredin the completely through-hardened bushings made of the steels Nos. 3and 4. It is understood from these results that the frequency ofquenching crack increases with the degree of through hardening.

[0111] FIGS. 8 to 12 show the distributions of hardness in the crosssections of the crawler belt bushings (Dimension D) produced from thesteel materials Nos. 1, 2, 4, 5 and 6 when inner circumferential surfacecooling and outer circumferential surface cooling were started at thesame time and when inner circumferential surface cooling was started inadvance of outer circumferential surface cooling. In the figures, thenumber of bushings having quenching cracks per 10 bushings (crack ratio)is indicated, from which it is understood that quenching crack can beperfectly prevented by starting inner circumferential surface cooling 2seconds before outer circumferential surface cooling. The lead time forpreventing quenching crack is thought to be dependent on the thicknessof a crawler belt bushing to be treated. For instance, it has been foundthat quenching crack can be prevented in a small-sized crawler beltbushing (Dimension A) having a thickness of 8.3 mm, by setting the leadtime to about 1 second.

[0112] The bushing made of the steel No. 6 containing 1.34 wt % carbonwas through hardened and perfectly prevented from quenching crack bystarting inner circumferential surface cooling 8 seconds before outercircumferential surface cooling. As understood from FIGS. 10 to 12, thehardness of the outer circumference surface hardened layer is 700 to 850Hv, which is equal to or more than those of crawler belt bushingstreated by carburization heating and therefore it is obvious that thewear resistance of the outer circumference of the bushing has beensignificantly improved. Also, quenching crack was checked when thecrawler belt bushing (Dimension D) manufactured from the steel No. 4 wasquenched by starting outer circumferential surface cooling in advance ofinner circumferential surface cooling. In this case, quenching crack wasprevented substantially similarly to the case of quenching of thebushing No. 6 by firstly starting inner circumferential surface cooling.

[0113] FIGS. 13 to 18 show the distributions of hardness in the crosssections of bushings having the respective dimensions when innercircumferential surface cooling and outer circumferential surfacecooling were started at the same time and when inner circumferentialsurface cooling was started in advance of outer circumferential surfacecooling.

[0114] The following is understood from the results of the above tests.

[0115] (1) Even when treating a steel which is usually fully hardened atits core by simultaneously starting inner circumferential surfacecooling and outer circumferential surface cooling, a soft layer can beformed within the core region at a cross-sectional position closer tothe inner circumferential surface so as to form a U-shaped distributionof hardness, by utilizing the mass effect of the wall of the bushingobtained by employing different starting times for inner circumferentialsurface cooling and outer circumferential surface cooling. Therefore,the same effect can be expected by making the cooling power of the innercircumferential surface different from the cooling power of the outercircumferential surface, more specifically, by making the cooling powerof the inner circumferential surface lower than that of the outercircumferential surface.

[0116] (2) It is understood, from the relationship between the degree ofthrough hardening and the frequency of quenching crack with respect tothe bushings having the dimension D, that bushings which are throughhardened by the simultaneous cooling can be perfectly prevented fromquenching crack by imparting hardness to the bushings so as to have aU-shaped distribution of hardness.

[0117] (3) Further, a steel, which is not usually through hardened bythe simultaneous cooling, can be more deeply hardened by the effect ofthe increased cooling rate of the outer circumferential surface. Theincrease in the cooling rate can be achieved by reducing heat capacityin the core by the advance cooling from the inner circumferentialsurface and achieved by the later cooling from the outer circumferentialsurface.

[0118]FIG. 19 shows the relationship between the lead time of advancecooling from the inner circumferential surface and the hardened depthsof the inner and outer circumferential surfaces of the bushing havingthe dimension C. It is understood from this figure that when the leadtime is in a certain range, the maximum hardened depth of the outercircumferential surface is obtained. In view of wear life, it ispreferable that the hardened depth of the outer circumferential surfacebe at least 1.1 times the hardened depth of the inner circumferentialsurface or more. As seen from the data shown in FIGS. 13 to 18, themaximum hardened depth of the outer circumferential surface in theinvention is about twice the hardened depth of the inner circumferentialsurface. This means that excellent wear life can be achieved by theinvention. FIG. 20 shows the relationship between the lead time ofadvance cooling from the outer circumferential surface and the hardeneddepths of the inner and outer circumferential surfaces of the bushinghaving the dimension C. Oppositely to the result shown in FIG. 19, thehardened depth of the inner circumferential surface can be increased bystarting outer circumferential surface cooling in advance of innercircumferential surface cooling. This process is suitably used in theproduction of wear-resistant, strong pipe products having a bore fordelivering earth/sand and slurry therethrough.

[0119] Although the hardened depth of the outer circumferential surfacecan be increased by the conventional heat treatment employing thermalrefining and induction hardening in combination, this method needs tocarry out thermal refining and induction hardening for each of the innerand outer circumferential surfaces, which is economicallydisadvantageous.

[0120] (Wear Resistance Test on Crawler Belt Bushings)

[0121] A crawler belt bushing (Dimension C; Composition No. 8; thehardened depth of the outer circumferential surface 5.3 mm) treated bythe process according to Example 1 and a conventional carburized bushing(Dimension C; quality=SCR420H; hardened depth=2.4 mm) were respectivelymounted on a crawler belt (D50, produced by Komatsu) of a bulldozer andused in soil dressing in a rice field. FIG. 21 shows the results of thewear tests. After 2,200 hour operation, the conventional bushing wasworn by 5 mm, whereas the bushing according to the invention was worn by2.8 mm.

[0122] It took about 3,600 operating hours for the bushing of theinvention to reach the critical wear amount of 5 mm, which is aconsiderable improvement in wear life. When considering the fact thatthe critical wear amount is about one half the thickness of a bushingand that the hardened layer of the outer circumferential surface formedby the thermal treatment of the invention reaches the substantial coreregion as seen from FIG. 13 to 18, the invention has brought about aconsiderable improvement in wear life. The test specimen of theinvention has high hardness in the outer circumferential surfacehardened layer (see FIG. 15) and therefore a low wear rate in thehardened layer, compared to the conventional carburized bushing, so thatit has proved to be excellent in wear resistance.

[0123] (Collapse Fatigue Test on Crawler Belt Bushings)

[0124]FIG. 22 illustrates a method for testing collapse fatigue. Abushing having the configuration shown in FIG. 6 was forced into acrawler belt link 8 and a load F that was about twice the weight of thevehicle was repeatedly imposed on a specified position (20 mm away fromthe end face of the link in this example) to check the number ofrepetitions, that is, how many times the load was imposed until thebushing was brought to breakage. FIG. 23 shows the number of repetitionswhich brought the bushing to breakage when loads of 2 to 37.5 tons wereapplied to the bushings having the dimension C prepared according tothis example. Three bushings, that is, the above conventional carburizedbushing, the bushing treated by the time difference quenching of theinvention, and the bushing treated by the simultaneous quenching werecompared in terms of fatigue strength. It is apparent from thecomparison that the bushing according to the invention exhibits higherfatigue strength than the conventional carburized bushing. Similarcomparison was made using large-sized specimens having the dimension D,from which it was found that the bushing treated by the time differencequenching of the invention was superior in fatigue strength to theconventional carburized bushing and to the non-through-hardened bushingtreated by the simultaneous quenching.

[0125] (Impact Fatigue Test)

[0126]FIG. 24 shows a method for testing impact fatigue strength. Thecrawler belt bushing treated by the thermal treatment according toExample 1 was forced into a crawler belt link and repeatedly struck witha hammer, causing impact load so that the stresses exerted on the innercircumferential surface of the bushing were to two, three, and fourtimes the weight of the vehicle. The number of repetitions which broughtthe bushing to breakage was checked to measure its impact fatigueproperties. For comparison, a specimen (Vickers Hardness Hv=about 280)was prepared in the following way: A bushing manufactured from aSCrB440H boron steel was thermally refined (oil quenching at 850° C. andtempering at 500° C. for 3 hours) and after induction hardening, thebushing was tempered at 180 C for 3 hours, thereby obtaining a hardeneddepth of about 3.5 mm at the inner and outer circumferential surfaces.

[0127]FIG. 25 shows the results of the measurements. It is apparent fromthis figure that the bushings according to the invention are improved inimpact strength over the conventional carburized bushings. Probably,this is attributable to the facts that a grain boundary layer orimperfectly hardened layer exists in the inner circumferential surfaceof the conventional carburized bushings and that the carburized bushingshave higher surface carbon content (about 0.8 wt %) and higher surfacehardness. This means that the impact fatigue strength of the bushing ofthe invention can be increased by adjusting the hardness of the innercircumferential surface to increase toughness. FIG. 26 shows therelationship between the hardness of the inner circumferential surfaceand the number of impact ruptures when the bushing of the invention wasinduction tempered from the inner circumferential surface. As apparentfrom the figure, when the surface hardness Hv is 550 to 600, the optimumstrength can be obtained. For example, when the bushing of the inventionhas a surface hardness Hv of 400, it has higher strength than theconventional carburized bushing, but excessive distortion occurred atthe inner circumference of the bushing after the test. This distortionmay cause interference with crawler belt pins, resulting in galling andabrasion. Therefore, the hardness Hv for the invention is preferably 450or more. The upper limit of hardness for the invention is notparticularly specified by the comparison with the conventionalcarburized bushing, but may be as high as the surface hardness of thecarburized bushing (Hv=up to 750). However, in order to achieve theoptimum impact strength properties, the hardness Hv of the innercircumferential surface of the invention is preferably limited to about650.

Example 2

[0128] A bushing was hardened under the hardening conditions shown inTABLE 3, using the hardening system shown in FIG. 3. The dimension andcomposition of the bushing used in this example were D, and No. 3 (thesteel of No. 3 is usually through hardened by simultaneous quenchingfrom the inner and outer circumferential surfaces), respectively. Thedifference between the position of the inner circumferential surface ofthe bushing upon which cooling water from the inner circumferentialsurface cooling nozzle 5 strikes and the position of the outercircumferential surface upon which cooling water from the outercircumferential surface cooling nozzle 6 strikes was adjusted to 30 mm.When the moving speed was 5 mm/sec., the lead time of innercircumferential surface cooling was about 6 sec. Further, thetemperature of the induction heating was adjusted so as to obtain atemperature of about 920° C. at the outer circumferential surface and atemperature of about 850° C. at the inner circumferential surface. TABLE3 INDUCTION HEATING SCAN QUENCHING CONDITIONS HEATING · QUENCHINGCONDITIONS FREQUENCY (KHz) 1 OUTPUT (KW) 85 FEED RATE (mm/sec) 5.0COOLING METHOD (INNER, OUTER) WATER SPRAY

[0129]FIG. 27 shows the distribution of hardness in the cross section ofthe bushing tempered at 140° C. for one hour after quenching. As seenfrom this figure, the hardened depth of the outer circumferentialsurface of the bushing according to this example is significant likeExample 1 in which the time difference quenching is carried out afterheating in a furnace and it is therefore understood that the bushing ofthis example has been improved in wear life.

[0130] Although the induction heating coil is disposed on the side ofthe outer circumferential surface of the bushing in this example, it maybe disposed on the side of the inner circumferential surface of thebushing. However, it is preferable to carry out induction heating fromthe outer circumferential surface side, taking the operating performanceof the hardening system into account.

EXAMPLE 3

[0131] TABLE 4 shows the steel compositions of the bushings used inExample 3. The bushings used in this example have the dimension C (seeTABLE 2). The scan induction hardening system shown in FIG. 5 wasemployed in this example. This hardening system comprises two highfrequency coils 8, 9 aligned vertically for heating the bushing from theouter circumferential surface side; a nozzle 10 for cooling the innercircumferential surface of the bushing; and a nozzle 11 for cooling theouter circumferential surface of the bushing. The scan heating/quenchingis carried out by the relative movement of the bushing, coils 8, 9 andnozzles 10, 11 so that the treatment proceeds upwardly, starting fromthe lower part of the bushing. The inner circumferential surface nozzle10 is designed to have an adequate jetting angle with respect a normalline to the inner circumferential surface in order to allow waterpresent in the inner circumference to flow toward the lower portion ofthe bushing without being trapped. There is provided, near the lower endof the bushing, a shield plate for dividing a flow of cooling water usedfor inner circumferential surface cooling and a flow of cooling waterused for outer circumferential surface cooling from each other. Disposednear the upper end of the bushing is a cap for dividing a flow ofcooling water used for inner circumferential surface cooling and a flowof cooling used for outer circumferential surface cooling from eachother. A high frequency power source of 6 KHz, giving an output of 50 KWwas used and hardening tests were conducted with the output of about 27to 32 KW. Some bushing specimens were subjected to low-temperaturetempering at 140° C. for 3 hours after quenching. Some were subjected toentire induction heating from the outer circumference surface, using thesame high frequency power source, and when the temperature of the innercircumferential surface has reached 850° C., inner circumferentialsurface cooling was started while continuing the induction heating. 6seconds later, the heating was finished to start outer circumferentialsurface cooling. TABLE 4 No. C Si Mn Cr Mo P S Al  4 0.61 0.19 1.17 — —0.018 0.018 0.037 11 0.75 0.18 0.61 1.02 0.16 0.010 0.012 0.028 12 1.450.28 0.58 0.82 — 0.009 0.008 0.031

[0132] FIGS. 28 to 30 show the distribution of hardness in the crosssections of the crawler belt bushings manufactured from the steels Nos.4, 11 and 12 (see TABLE 4) treated by the scan hardening, in the casewhere inner circumferential surface cooling and outer circumferentialsurface cooling are started at the same time and in the case where theinner circumferential surface is first cooled with the outercircumferential surface cooling nozzle being shifted downward, and 6 to10 seconds later, outer circumferential surface cooling is started. Itshould be noted that the crawler belt bushings associated with FIG. 30were thermally refined, by heating at 1,020° C. for 30 minutes,oil-quenching and then tempering at 600° C. for 1 hour.

[0133] The following facts are understood from the results of the abovetests.

[0134] (1) Even when treating a steel which is usually hardened throughits cross section by simultaneous quenching from the inner and outercircumferential surfaces, a soft layer can be formed within the core ata cross-sectional position closer to the inner circumferential surfaceso as to form a U-shaped distribution of hardness, by carrying outinduction heating from the outer circumferential surface during theadvance cooling, that is, the cooling from the inner circumferentialsurface.

[0135] (2) Steels, which are usually hardened through their crosssections by simultaneous quenching from the inner and outercircumferential surfaces, can be perfectly prevented from quenchingcrack by the hardening process of the invention so that steels havingvery high carbon content can be used for the bushings.

[0136] (3) Even when treating a steel, which is not usually hardenedthrough its cross section by simultaneous quenching from the inner andouter circumferential surfaces, hardened depth can be increased by theeffect of the increased cooling rate of the outer circumferentialsurface. The cooling rate of the outer circumferential surface isincreased by reducing heat capacity in the core region through theadvance cooling from the inner circumferential surface and by the latercooling from the outer circumferential surface.

[0137] (4) Since this example uses steels having high carbon content forthe bushings, the hardness of the quench hardened layer of each bushingis substantially as high as the hardness of the carburized bushing.Further, the hardened layer of each bushing is deeper than that of thecarburized bushing, and therefore it is understood that the bushings ofthis example have been considerably improved in wear life (the wear lifeof a bushing is evaluated by the time taken until about half thicknessof the bushing has been worn away).

[0138]FIG. 31 shows the distribution of hardness in the cross section ofthe crawler belt bushing having the composition No. 11 when after entireinduction heating, the inner circumferential surface was first cooledand 10 seconds later, the outer circumferential surface was cooled. Theresult shown in this figure is substantially similar to the dataobtained from FIG. 29 and it is understood that quench hardened layersare formed in this bushing by the same heating/cooling mechanism as thatof the scan hardening described earlier.

[0139]FIG. 32 shows the distribution of hardness in the cross section ofthe crawler belt bushing having the composition No. 11 when after entireinduction heating, the inner circumferential surface was first cooledwhile heating the outer circumferential surface with heating power (13KW) which was about one-third the heating power of the entire heatingand 10 seconds later, the outer circumferential surface was cooled. Inthis case, the hardened depth of the inner circumferential surface couldbe increased, on the contrary to the result shown in FIG. 31. Such abushing is suited for use in the production of wear-resistant, strongpipe products with a bore for delivering earth/sand and slurrytherethrough.

[0140] (Impact Fatigue Test)

[0141] The crawler belt bushings were treated by the scan inductionhardening of this example and then tempered at 180° C. for 3 hours.Then, these bushings were respectively forced into a crawler belt linkand tested, using the impact fatigue tester shown in FIG. 24.Specifically, the bushings were repeatedly struck with a hammer, causingimpact load so that the stresses exerted on the respective innercircumferential surfaces were equivalent to two, three, and four timesthe weight of the vehicle. The number of repetitions which brought eachbushing to breakage was checked to measure the impact fatigueproperties. For comparison, specimens were prepared in the followingway: For preparing a specimen (Vickers Hardness Hv=about 280), a bushingmanufactured from a SCrB440H boron steel was thermally refined (oilquenching at 850° C. and tempering at 500° C. for 3 hours) and theninduction hardened to obtain a hardened depth of about 3.5 mm at theinner and outer circumferential surfaces. Another specimen was preparedby carburization-hardening a bushing made of SCr420H steel at 930° C.,and then tempering it at 180° C. for 3 hours so as to obtain a hardeneddepth of 2.5 mm.

[0142] The results of the measurements are shown in FIG. 33. As seenfrom this figure, the bushings prepared according to the invention areimproved in impact strength over the conventional carburized bushings.This is presumably attributable to the facts that a grain boundary layeror imperfectly hardened layer exists in the inner circumferentialsurface of the conventional carburized bushings and that the carburizedbushings have higher surface carbon content (about 0.8 wt %) and highersurface hardness. This means that the impact fatigue strength of thebushings of the invention can be increased by adjusting the hardness ofthe inner circumferential surface to increase toughness. It isconventionally known that when the surface hardness Hv of the innercircumferential surface is 500 to 600, the optimum strength can beobtained. For example, when the bushing of the invention has a surfacehardness Hv of 400, it has higher strength than the conventionalcarburized bushings, but distortion occurred at the inner circumferenceafter the test. This distortion is a cause of interference with crawlerbelt pins. Therefore, the hardness for the invention is preferably 450or more. The upper limit of hardness Hv for the invention is notparticularly specified by the comparison with the conventionalcarburized bushing, but may be as high as the surface hardness of thecarburized bushing (Hv=up to 750). However, in order to achieve theoptimum impact strength properties, the hardness Hv of the innercircumferential surface of the invention is preferably limited to about650. It is important particularly for the busing having the compositionNo. 12 and manufactured according to the invention in which cementitegrains are dispersed, that cementite does not precipitate in the prioraustenite grain boundary and that the soft layer formed in the core ismostly composed of a bainitic structure having granular cementitedispersed therein.

EXAMPLE 4

[0143] In Example 4, scan hardening is carried out along the bushingfrom the bottom to the top, using the scan induction hardening system(shown in FIG. 5) employed in Example 3. Since the heat input at thelower end of the bushing in the initial stage is small, the highfrequency coils 8, 9 are interrupted for a specified length of time.When scan quenching is carried out by use of the inner circumferentialsurface cooling nozzle 10 and the outer circumferential surface coolingnozzle 11, the level at which water jetted from the nozzle 10 strikesand the level at which water jetted from the nozzle 11 strikes are madeto be equal to each other within specified zones close to the lower andupper end faces, respectively, of the bushing. With this arrangement,the bushing is hardened through its entire cross section at the lowerand upper ends thereof.

[0144] Reference is now made to FIG. 34 to describe the steps of thehardening operation according to Example 4in detail.

[0145] In the initial stage shown in FIG. 34(a), the high frequencycoils 8, 9 are stationary and heating is carried out until the innercircumferential surface of the bushing reaches the austenitictemperature range. Then, the high frequency coils 8, 9 are moved upwardat constant speed, being followed by the inner and outer circumferentialsurface cooling nozzles 10, 11. At this stage, the level at which thecoolant jetted from the nozzle 10 strikes upon the bushing surface andthe level at which the coolant jetted from the nozzle 11 strikes uponthe bushing surface are equal to each other. At that time, the innercircumferential surface and outer circumferential surface aresimultaneously cooled within the region having a specified height h₁from the lower end of the bushing, as shown in FIG. 34(b). Since theinner and outer circumferential surfaces are simultaneously cooled inthe lower region of the bushing, the bushing can be hardened through itscross section in this region, if the steel of the bushing has enoughhardenability.

[0146] As shown in FIG. 34(c), after the striking level of the coolantsfrom both nozzles 10, 11 has reached a specified level, the upwardmovement of the outer circumferential surface cooling nozzle 11 isstopped, while the inner circumferential surface cooling nozzle 10 beingcontinuously moved upward at a specified speed in synchronous relationwith the high frequency coils 8, 9. At the time when the differencebetween the striking level of the coolant from the nozzle 10 and thestriking level of the coolant from the nozzle 11 becomes equal to aspecified distance h₂, the nozzle 11 is moved upward synchronously withthe nozzle 10 at the same speed as that of the nozzle 10. In this way,the cooling from the inner circumferential surface advances by the timethat is obtained by dividing the level difference h₂ by the moving speedof the nozzles 10, 11. Thanks to this advance cooling from the innercircumferential surface, the temperature of the bushing at the center inits cross section decreases to such a temperature range in whichhardening does not occur, and as a result, an imperfectly hardened layeris created inside the wall of the bushing.

[0147] At the time when the striking level of the coolant from thenozzle 10 has reached a position having a specified distance h₃ (=h₁)from the upper end of the bushing, the nozzle 10 is stopped, as shown inFIG. 34(d). Thereafter, the nozzle 11 is continuously moved upward. Whenthe striking level of the coolant from the nozzle 10 and the strikinglevel of the coolant from the nozzle 11 become equal to each other asshown in FIG. 34(e), the nozzles 10, 11 are synchronously moved towardthe upper end of the bushing while keeping the coolant striking levelsof both nozzles the same, as shown in FIG. 34(f). When reached the upperend, the nozzles are stopped, while cooling is continued until thebushing is completely cooled.

[0148]FIG. 35 shows three entire hardened patterns when a 15 mm-thickcrawler belt bushing for bulldozers is hardened. Specifically, (a) isthe pattern obtained by carburization hardening; (b) is the patternobtained by the time difference hardening incorporating inductionheating, with a constant positional relationship between the coolingnozzles; and (c) is the pattern obtained by the hardening process ofExample 4. The bushings used herein are made of a S55C steel having a DIvalue (indicating the hardenability of steel) of 1.5, and the timedifference (i.e., lead time) between the inner circumferential surfacecooling and the outer circumferential surface cooling is 6 seconds. Asseen from FIG. 35, in the bushing treated by the hardening process ofExample 4, hardened layers are formed at both ends, and the depth of theouter circumference is greater than the depth of the innercircumference. In addition, the depth of the outer circumference ofExample 4 is greater than that of the carburized bushing. In contrastwith this, the bushing (b) treated by the time difference hardening witha constant nozzle positional relationship has a hardened layer at theupper end face only, with the internal imperfectly hardened layerextending throughout the lower end of the bushing.

1. A crawler belt bushing producing method, wherein a workpiece of acrawler belt bushing made of a medium or high carbon steel or a mediumor high carbon low alloy steel is heated to a quenching temperature ormore; by use of a hardening system which can independently start outercircumferential surface cooling and inner circumferential surfacecooling, the workpiece is first cooled from either one of its outercircumferential surface and inner circumferential surface and thencooling is carried out from the other circumferential surface so thatthe workpiece is hardened through its entire thickness; and then theworkpiece is tempered.
 2. A crawler belt bushing producing methodaccording to claim 1 , wherein said hardening system is designed to havea partition between a cooling medium for the inner circumferentialsurface and a cooling medium for the outer circumferential surface,taking into account the flows of the cooling media, such that thecooling media do not interfere with each other during cooling of theworkpiece.
 3. A crawler belt bushing producing method according to claim1 , wherein after the workpiece is substantially uniformly and entirelyheated to a quenching temperature by furnace heating and/or inductionheating, the workpiece is first cooled from either one of the innercircumferential surface and outer circumferential surface, and one ormore seconds later, the workpiece is cooled from the othercircumferential surface.
 4. A crawler belt bushing producing methodaccording to claim 1 , wherein while scan induction heating in the axialdirection of the workpiece being carried out from either one of theinner circumferential surface and outer circumferential surface, coolingof the heated surface with a spray is started from either one of theinner circumferential surface and outer circumferential surface, and oneor more seconds later, cooling of the heated surface with a spray isstarted from the other circumferential surface.
 5. A crawler beltbushing producing method according to claim 1 , wherein said temperingprocess is carried out at 140 to 300° C.
 6. A crawler belt bushingproducing method according to claim 1 , wherein the hardness of theinner circumferential surface is adjusted to Rockwell hardness (C type)HRC 45 to 55 by said tempering process.
 7. A crawler belt bushingproducing method according to claim 6 , wherein said adjustment of thehardness of the inner circumferential surface is carried out by stoppingthe induction tempering from the inner circumferential surface and/orthe cooling from the inner circumferential surface during quenchingoperation, earlier than the cooling from the outer circumferentialsurface, thereby allowing self-tempering of the inner circumferentialsurface.
 8. A crawler belt bushing producing method wherein, after aworkpiece of a crawler belt bushing made of steel is heated to aquenching temperature, (a) the cooling rate of the outer circumferentialsurface of the workpiece is increased by first cooling of the workpiecefrom its inner circumferential surface in order to reduce heat capacityat the core of the workpiece and by second cooling of the workpiece fromits outer circumferential surface which is started a certain time afterthe first cooling and/or (b) the cooling rate of the outercircumferential surface of the workpiece is increased by first coolingof the workpiece from its inner circumferential surface in order topartially make the core of the workpiece unhardenable by utilizing themass effect of the wall of the workpiece and by second cooling of theworkpiece from its outer circumferential surface which is started acertain time after the first cooling, whereby a soft layer is formedwithin the core of the workpiece at a cross-sectional position closer tothe inner circumferential surface and the hardened depth of the outercircumferential surface is made to be greater than the hardened depth ofthe inner circumferential surface, said processes (a) and (b) beingcarried out within one cycle of quenching operation, using a hardeningsystem capable of performing inner circumferential surface cooling andouter circumferential surface cooling.
 9. A crawler belt bushingproducing method according to claim 8 , wherein said hardening system isdesigned to have a partition between a cooling medium for the innercircumferential surface and a cooling medium for the outercircumferential surface, taking into account the flows of the coolingmedia, such that the cooling medium for the inner circumferentialsurface does not interfere with the outer circumferential surface duringthe first cooling from the inner circumferential surface.
 10. A crawlerbelt bushing producing method according to claim 9 , wherein saidcooling media are quenching oil, water, water-soluble quenching liquidor water fog and wherein said cooling from the inner circumferentialsurface is jet cooling which uses a spray for substantially uniformlycooling the inner circumferential surface.
 11. A crawler belt bushingproducing method according to claim 8 , wherein after the workpiece issubstantially uniformly and entirely heated to a quenching temperatureby furnace heating and/or induction heating, quenching is carried outusing said hardening system.
 12. A crawler belt bushing producing methodaccording to claim 8 , wherein while scan induction heating in the axialdirection of the workpiece being carried out from either one of theinner and outer circumferential surfaces, scan quenching is carried outby first cooling the workpiece from the inner circumferential surfaceand then cooling the workpiece from the outer circumferential surface,under the condition that the temperatures of the inner and outercircumferential surfaces are quenching temperatures equal to or higherthan the transformation temperatures of A1, A3 and/or Acm.
 13. A crawlerbelt bushing producing method according to claim 12 , wherein said scanquenching during the induction heating is carried out such that theworkpiece, an induction heating coil, an inner circumferential surfacecooling nozzle and an outer circumferential surface cooling nozzle arerelatively moved in the axial direction of the workpiece and theworkpiece is rotated substantially about its mean axis.
 14. A crawlerbelt bushing producing method according to claim 8 or 9 , whereincooling from the inner circumferential surface and cooling from theouter circumferential surface are carried out at substantially the sametime at least within specified regions close to the upper end face andlower end face, respectively, of the workpiece, so that the upper andlower end faces are through hardened.
 15. A crawler belt bushingproducing method wherein the workpiece hardened by the bushing producingmethod set forth in claim 8 is tempered at 140° C. to 350° C.
 16. Acrawler belt bushing producing method according to claim 8 , whereinsaid workpiece is made of a steel having a carbon content equal to thoseof medium carbon steels and/or eutectoid steels, which is 0.35 wt % ormore, and having an alloy content within the range of DI values withwhich the workpiece is through hardened by simultaneous cooling of theinner and outer circumferential surfaces and with which the hardeneddepth obtained by cooling from the inner circumferential surface only isabout one half the thickness of the workpiece.
 17. A crawler beltbushing producing method, wherein the inner circumferential surface ofthe workpiece hardened by the bushing producing method set forth inclaim 8 is tempered by induction tempering so as to have a surfacehardness of Vickers hardness Hv 450 to
 650. 18. A crawler belt bushingproducing method wherein, after a workpiece of a crawler belt bushingmade of steel is induction heated from its outer circumferential surfacesuch that at least the temperature of the inner circumferential surfaceof the workpiece is raised to a quenching temperature, a series ofquenching operation comprising: (a) firstly cooling the workpiece fromthe inner circumferential surface; (b) cooing the workpiece from theinner circumferential surface while heating the workpiece from the outercircumferential surface; and (c) cooling the workpiece from the outercircumferential surface, is performed so as to form quench hardenedlayers which extend toward the wall core of the workpiece from the outercircumferential surface and from the inner circumferential surfacerespectively and form a soft, imperfectly hardened layer between saidquench hardened layers.
 19. A crawler belt bushing producing methodwherein, while a workpiece of a crawler belt bushing made of steel beingheated from its outer circumferential surface by scan induction heatingin the axial direction of the workpiece, using at least two verticallyaligned, induction coils, (a) the temperature of the innercircumferential surface of the workpiece is raised to a quenchingtemperature equal to the transformation temperatures of A1, A3 and/orAcm or more; (b) the workpiece is partially heated from the outercircumferential surface by the induction coils while carrying out firstcooling from the inner circumferential surface; and (c) the workpiece isthen cooled from the outer circumferential surface; whereby the innerand outer circumferential surfaces are quench hardened so as to besubstantially martensitic, said processes (a), (b) and (c) being carriedout at a certain position of the workpiece.
 20. A crawler belt bushingproducing method according to claim 18 or 19 , wherein cooling from theinner circumferential surface and cooling from the outer circumferentialsurface are carried out at substantially the same time at least withinspecified regions close to the upper end face and lower end face,respectively, of the workpiece, so that the upper and lower end facesare through hardened.
 21. A crawler belt bushing producing methodaccording to claim 18 or 19 , which uses a hardening system designed tohave a partition between a cooling medium for the inner circumferentialsurface and a cooling medium for the outer circumferential surface,taking into account the flows of the cooling media, such that thecooling medium for the inner circumferential surface does not interferewith the outer circumferential surface during the first cooling from theinner circumferential surface.
 22. A crawler belt bushing producingmethod according to claim 19 , wherein said scan quenching by inductionheating is carried out such that the workpiece, an induction heatingcoil, an inner circumferential surface cooling nozzle and an outercircumferential surface cooling nozzle are relatively moved and theworkpiece is rotated substantially about its mean axis.
 23. A crawlerbelt bushing producing method according to claim 21 , wherein saidcooling media are quenching oil, water, water-soluble quenching liquidor water fog and wherein said cooling from the inner circumferentialsurface is jet cooling which uses a spray for substantially uniformlycooling the inner circumferential surface.
 24. A crawler belt bushingproducing method, wherein the workpiece hardened by the bushingproducing method according to claim 18 or 19 is tempered at 140 to 350°C.
 25. A crawler belt bushing wherein quench hardened layers are formedso as to extend toward its wall core from its outer circumferentialsurface and from its inner circumferential surface respectively and asoft, imperfectly hardened layer is formed between said quench hardenedlayers, said quench hardened layers and said soft layer being formedsuch that the quench hardened layer of the outer circumferential surfacehas a depth greater than the depth of the quench hardened layer of theinner circumferential surface, by: (a) increasing the cooling rate ofthe outer circumferential surface by first cooling of the workpiece fromits inner circumferential surface in order to reduce heat capacity atthe core and by second cooling of the workpiece from its outercircumferential surface which is started a certain time after the firstcooling and/or (b) increasing the cooling rate of the outercircumferential surface by first cooling of the workpiece from its innercircumferential surface in order to partially make the core unhardenableby utilizing the mass effect of the wall of the workpiece and by secondcooling of the workpiece from its outer circumferential surface which isstarted a certain time after the first cooling, the structure betweensaid quench hardened layers being composed of one or more structuresselected from ferrite, pearlite, bainite and martensite which areprecipitated during cooling from the quenching temperature, said bushingbeing low temperature tempered.
 26. A crawler belt bushing according toclaim 25 , wherein the hardened depth of the outer circumferentialsurface is not less than 1.1 times the hardened depth of the innercircumferential surface.
 27. A crawler belt bushing according to claim25 or 26 , which is made of a steel having a carbon content equal tothose of medium carbon steels and/or eutectoid steels, which is 0.35 wt% or more and having an alloy content within the range of DI values withwhich the bushing is through hardened by simultaneous cooling of theinner and outer circumferential surfaces and with which the hardeneddepth obtained by cooling from the inner circumferential surface only isabout one half the thickness of the bushing.
 28. A crawler belt bushingaccording to claim 25 , which is tempered at high temperature such thatthe quench hardened layer of the inner circumferential surface hashigher hardness than the quench hardened layer of the outercircumferential surface and wherein the surface hardness of the quenchhardened layer of the inner circumferential surface is adjusted toVickers hardness Hv 450 to
 650. 29. A crawler belt bushing according toclaim 25 , which is through hardened at its upper and lower ends.
 30. Acrawler belt bushing having a carbon content of 0.35 to 2.0 wt %,containing at least one of the alloying elements of Mn, Si, Cr, Mo andNi, and made by a method in which a bushing workpiece made of steel,which is through hardened by simultaneous cooling from the outer andinner circumferential surfaces of the workpiece, is induction heatedfrom the outer circumferential surface so as to raise at least thesurface temperature of the inner circumferential surface to a quenchingtemperature, and thereafter, a series of quenching operation comprising:(a) firstly cooling the workpiece from the inner circumferentialsurface; (b) heating the workpiece from the outer circumferentialsurface while cooing the workpiece from the inner circumferentialsurface; and (c) then, cooling the workpiece from the outercircumferential surface, is performed so as to form quench hardenedlayers which extend toward the wall core of the workpiece from the outercircumferential surface and from the inner circumferential surfacerespectively and form a soft, imperfectly hardened layer between saidquench hardened layers, said soft layer between the quench hardenedlayers being composed of one or more structures selected from ferrite,pearlite, bainite and martensite which are precipitated during coolingfrom the quenching temperature and which contain or do not containgranular cementite dispersed therein.
 31. A crawler belt bushingaccording to claim 30 , wherein the hardened depth of the outercircumferential surface is not less than 1.1 times the hardened depth ofthe circumferential surface.
 32. A crawler belt bushing according toclaim 30 , which is tempered at 140 to 350° C. after quenching.
 33. Acrawler belt bushing according to claim 30 , which is through hardenedat its upper and lower ends.